Math Monday: Mommy Math Jacket Extender

MR.A may be our third little bundle, but it's our first (real) winter with a wee one!
Though there are loads of lovely commercial coat covers and extensions for baby wearer's,
http://www.snugglebugz.ca/deluxe-coat-extension-black.html
https://www.babyandbeyond.ca/maternity-motherhood-fashion-accessories/makemybellyfit-zip-in-extender-makemybellyfit-zip-in-extender.html
I scoured several tutorials (poncho's, carrier covers, jacket extensions, husbands hand-me-downs!) before making up a design that could be sewn at home.
http://mamanloupsden.com/2016/09/05/babywearing-solutions-for-fall-winter/
The total came to a mere $10 (material plus zipper) and only several small headache's.

I started by measuring the length and width of MR.A in the carrier with my coat wrapped around him as far as possible.
Since I was inserting elastic in the bottom under his bum, I made it slightly wider than the top, where his neck would come to. I also sketched a hood, from the fleece one-piece he wears courtesy of Coz.
The hood would allow me to attach a zipper the length of my jacket zipper, as his neck and head only come 3/4 of the way up.

Once I mapped out measurements in chalk, I left a little seam allowance before snipping and sewing, starting with the elastic base, hood, attaching the hood and lastly the zippers.

In hindsight, I should have made a liner, sized to the original design before sewing, but the fleece is very forgiving and it doesn't look too shabby on the interior.